The present invention relates to a plasma generating method and apparatus.
A plasma generating method using high frequency electric discharge is used in the fields of dry-etching apparatus for microfabrication, plasma CVD apparatus or sputtering apparatus for forming thin films, ion implantation apparatus and the like. In such a plasma generating method, it is required to generate a plasma under a high vacuum in order to miniaturize the feature sizes or to control the film quality with high precision.
The following will discuss a dry etching method for microfabrication as an example of application of the plasma generating method.
The recent progress in the field of highly dense semiconductor integrated circuit s is bringing about great changes equivalent to those brought by the Industrial Revolution. The highly dense arrangement of a semiconductor integrated circuit has been achieved by miniaturization of element dimensions, improvements in devices, provision of large-area chips and the like. Element dimensions are now miniaturized to the extent of the wavelength of light. In lithography, the use of excimer laser or soft X-ray is taken into consideration. To realize micro-patterns, dry etching plays an important role as lithography does.
Dry etching is a proess technology for removing unnecessary parts of a thin film or a substrate with the use of chemical or physical reactions on the surface of a gas-solid phase of radicals, ions or the like present in a plasma. As dry etching, there is most widely used a reactive ion etching (RIE), according to which a sample is exposed to a high-frequency discharge plasma of a suitable gas, so that an etching reaction is generated on the sample surface to remove unnecessary parts thereof. Generally, the necessary parts or parts not to be removed of the sample surface, are to be protected by a photo-resist pattern serving as a mask.
For miniaturization, it is required to properly arrange ions in direction. In this connection, it is important to reduce ion scattering in the plasma. To properly arrange the ions in direction, it is effective to decrease the pressure in a plasma generating apparatus to increase the mean free path of the ions. However, when the pressure in the plasma chamber is decreased, this presents the problem that discharge of high frequency hardly occurs.
In view of the foregoing, there has been developed a method of applying a magnetic field to a plasma chamber to facilitate discharge, e.g., a magnetron reactive ion etching technology, an electron cyclotron resonance etching technology (ECR), or the like.
FIG. 17 is a schematic diagram of a reactive ion etching apparatus using conventional magnetron discharge. Reactive gas is introduced into a metallic chamber 51 through a gas controller 52. The pressure in the chamber 51 is controlled to a suitable value by an exhaust system 53. An anode 54 is disposed at an upper part of the chamber 51, and a sample stage 55 serving as a cathode is disposed at a lower part of the chamber 51. An RF power supply 57 is connected to the sample stage 55 through an impedance matching circuit 56, so that high frequency discharge takes place between the sample stage 55 and the anode 54.
Disposed at the lateral sides of the chamber 51 are two pairs of AC electromagnets 58 of which phases are shifted by 90.degree., the AC electromagnets 58 of each pair being opposite to each other. By the two pairs of AC electromagnets 58, a rotational magnetic field is applied into the chamber 51 to facilitate discharge under a high vacuum. With such an arrangement, the rotational magnetic field causes electrons to present cycloid motions. This lengthens the motional passages of the electrons to increase the efficiency of ionization.
FIG. 18(a) shows an example of etching boron phosphorus glass with a conventional magnetron reactive ion etching apparatus or ECR (electron cyclotron resonance) etching apparatus. In FIG. 18(a), there are shown a Si substrate 60, boron phosphorus glass 61 and a photo-resist pattern 62.
Such a conventional apparatus disadvantageously provokes damages to devices as set forth in the following.
In the conventional magnetron reactive ion etching apparatus, the rotational magnetic field averages the uneven distribution of a plasma with the passage of time, causing the plasma ununiformity to be equalized. However, since the momentary densities of the plasma are not always uniform, the potentials locally differ from one another. Accordingly, when the conventional magnetron reactive ion etching apparatus is applied to a MOSLSI process, there is a possibility of a gate oxide layer being broken.
In the ECR etching apparatus, too, since the magnetic field is distributed in the radial direction of the chamber as shown in FIG. 18(b), the plasma densities are locally coarse and dense. This causes the etching source to be ununiform or produces local differences in potential.
In view of the foregoing, the present invention is proposed with the object of generating a highly dense plasma excellent in uniformity under a high vacuum.